The Invention relates to completing wells drilled into subterranean formations. More particularly it relates to methods of killing a well, that is preventing fluid from flowing from the formation into the wellbore, during completion or remediation. Most particularly, it relates to killing a well without damaging it, that is without decreasing subsequent production, especially after hydraulic fracturing while a well is being prepared for production.
There are many wellbore operations in which it is necessary to allow a well to be in fluid communication with the formation it penetrates for a period of time under conditions where fluid could flow between the formation and the well unless steps were taken to prevent this. For example, after hydraulic fracturing, matrix acidizing or acid fracturing, frac packing, or gravel packing is completed it might be several days before appropriate hardware is installed in the next step of the completion (for example an electric submersible pump). Some method, which we will call “well control”, must be used to prevent fluid flow into the well during that time. Typically, the well is at least partially filled with a “kill fluid” or a “kill pill”. This fluid is commonly dense enough that the column of fluid provides a hydrostatic pressure greater than the pressure of fluids in the formation. The well control fluid is also commonly viscosified to reduce flow, and may contain solids, such as fluid loss additives, that are designed to block fluid flow paths. One of the serious drawbacks to viscosifying a kill fluid or including fluid loss additives, is that one or both of these features may damage the well, the fracture, or the formation. This could necessitate an additional step to attempt to clean up the damage. It may be difficult or impossible to remove all of the fluid or all of the solids, especially if some of the fluid and/or some of the solids invade the fracture or the formation. Also, after some methods of downhole completion (for example after installation of an ESP) remedial treatments are not possible
The solids used in well control fluids are usually substantially insoluble, or sparingly or slowly soluble, materials (typically those also commonly used as fluid loss additives and/or filter cake components). Removal of the solids is typically accomplished either by a mechanical means (scraping, jetting, or the like), or by subsequent addition of a fluid containing an agent (such as an acid, a base, an oxidizer, or an enzyme) that dissolves at least a portion of the solids. These removal methods usually require a tool or addition of another fluid (for example to change the pH or to add a chemical). Sometimes the operator may rely on the flow of produced fluids, but this method requires fluid flow and often results in slow or incomplete solids removal. Sometimes a breaker can be incorporated in the well control fluid but these must normally be delayed (for example by esterification or encapsulation) and they are often expensive and/or difficult to place and/or difficult to trigger.